The present invention relates to imaging systems in which two or more complex signals of a region of interest may be combined to yield one or more interferometric images. More particularly, the invention is directed to a method and system for two-dimensional radiometric imaging of a planetary surface region of interest utilizing thermal radiation emitted by the region of interest.
Computed imaging systems are utilized in a wide variety of applications. Of particular interest here is the use of radio frequency antennas to collect complex signals employable to obtain high quality images of planetary surfaces.
Such complex images are typically obtained by overhead transmission/reflected receipt of pulses of energy at a predetermined frequency. In the latter regard, microwave radiation has been advantageously employed due to its ability to yield high resolution images in virtually all weather conditions and at all times (i.e., day and night).
While such systems have been utilized with success, they require the use of radiation signal transmission payloads on one or more aircraft or satellites (i.e., xe2x80x9cspace vehiclesxe2x80x9d). As may be appreciated, such transmission payloads add significant weight, complexity and cost to an imaging system. Additionally, the use of active transmitters entails significant attendant power requirements. Further, the active transmission of microwave signals toward a region of interest is detectable and may be undesired in certain applications.
In view of the foregoing, a primary objective of the present invention is to provide an improved imaging system and method that reduces imaging componentry payload and complexity on space vehicles utilized to collect imaging data. Related objectives are to reduce on-board power requirements and componentry costs associated with the obtainment of imaging data on space-borne vehicles.
Another important objective of the present invention is to provide a radiometric imaging system and method that is passive in nature and thereby avoids the active transmission of energy signals to an image region of interest to form a pixel image thereof.
An additional main objective of the present invention is to provide an imaging system and method that reduces the number of space vehicles and associated antennas necessary for generating high-resolution images.
Yet another objective of the present invention is to provide an imaging system and method that provides high-resolution images in inclement weather and day/night conditions.
The above objectives and additional advantages are realized by the present invention. To do so, the present inventors have recognized that even though thermal emissions from a planetary surface region of interest are of random phase and amplitude, such emissions may be assumed to be largely isotropic and mutually coherent at a receiving antenna (e.g., as received or time-shifted), and may be collected and processed in a manner that allows such randomness to be effectively removed. Relatedly, it has been recognized that thermal radiation collection and processing can be carried out in a manner that reduces the number of antennas necessary to yield high-resolution images. At the outset it should be noted that while the present invention is particularly apt for radiometric imaging applications, certain aspects may also be employable in active imaging arrangements.
The inventive system contemplates a plurality of space vehicles located in known relative positions over a planetary surface region of interest (ROI). At least a corresponding plurality of antennas are mounted on the space vehicles to collect radiation emissions from the ROI (e.g., thermal or blackbody radiation) and provide corresponding thermal emission signals. In turn, processor means (e.g., one or more signal processors) may be utilized (e.g., either on-board the space vehicles and/or more preferably at another location) to combine the thermal emission signals and obtain interferometric fringe signals employable to form a pixel image of the ROI. As will be appreciated, the formation and use of interferometric fringes effectively removes phase randomness from the collected signals.
In one aspect of the invention, the space vehicles may be spaced at different relative distances therebetween, wherein the collection antennas collectively define a xe2x80x9csparse aperturexe2x80x9d. For such purposes, the space vehicles may be located so that two or more of the antennas are horizontally and/or vertically offset from each other in relation to the imaged ROI during imaging. Such an arrangement allows the thermal emission signals obtained by the antennas to be processed in varying combinations, wherein each combination yields a different interferometric phase measurement based upon a corresponding different interferometric baseline. As such, the multiple different interferometric phase measurements can effectively xe2x80x9cfill-inxe2x80x9d an array of interferometric images employable in pixel image formation for the ROI. As will be appreciated, the differential spacing of antennas to collectively define a sparse aperture facilitates reduction of the overall number of space vehicles required to yield high-resolution ROI images.
In a further aspect of the invention, the space vehicles may be positioned in a xe2x80x9cnear-fieldxe2x80x9d imaging arrangement to collect thermal emissions from an ROI. That is, the space vehicles may be positioned so that the imaging center axes for at least two of the antennas define an angle xcex8 of at least about 2xc2x0 therebetween, and more preferably about 2xc2x0 and 15xc2x0 therebetween, depending upon the collection center frequency of the antennas. In the latter regard, the antennas may be provided to collect thermal emissions over a collection bandwidth of between about 1 MHz and 1 GHz with a center frequency of between about 1 GHz and 100 GHz. The establishment of a near-field imaging arrangement also facilitates the obtainment of high-resolution ROI images.
In one arrangement, a plurality of antennas may be mounted on a corresponding plurality of satellites located in a known constellation passing over an ROI to be imaged. More particularly, two or more satellites may be located in corresponding repeatable orbits having relatively small differences in eccentricity and/or inclination (e.g., Hill""s orbits), wherein the corresponding antennas are horizontally and/or vertically offset in a known geometry relative to the ROI for imaging. By way of example, four satellites may be positioned in known orbits to laterally define a repeatable Y-shaped pattern for sparse aperture imaging. Further, at least two of the satellites may be positioned so that the center imaging axes of the corresponding antennas mounted thereupon define an angle of about 2xc2x0 to 15xc2x0 therebetween, thereby yielding a near-field imaging arrangement. In earth imaging applications, the satellites may be disposed in low-earth orbits, wherein the satellites are placed at altitudes and spacings consistent with near-field operations.
In conjunction with noted aspects of the present invention, it should be recognized that the antennas should be provided in a spotlight mode (e.g., via gimbaled mounting) so that they remain pointed at an imaged ROI during an imaging, or xe2x80x9cdwellxe2x80x9d, time period. Further in this regard, the antennas should be provided to collect thermal emissions from overlapping portions of the ROI in a substantially simultaneous manner to maintain mutual coherence. In turn, the thermal emissions collected from the ROI at each of the antennas may be substantially simultaneously sampled at a predetermined frequency (e.g., at least the Nyquist rate) during a given dwell period, thereby yielding an ROI thermal emission data set comprising each thermal emission signal. Such ROI data sets are combinatively employed by the processor means for image formation.
In the latter regard, and in another general aspect of the present invention, the processor means may be provided to combine, or correlate, at least a first thermal emission signal (e.g., collected by the first antenna) with a complex conjugate of at least a second thermal emission signal (e.g., collected by a second antenna) to obtain at least a first xe2x80x9csimplexe2x80x9d interferometric fringe signal. As will be appreciated, it is generally preferable therefore to correlate a plurality of different pairs of thermal emission signals to obtain a plurality of different simple interferometric fringe signals. Further, and in order to enhance mutual coherence, it may be desirable in certain applications to time-shift one of the thermal emission signals of a given pair prior to correlation (e.g., in high bandwidth applications).
After formation, each simple interferometric fringe signal(s) may be low-pass filtered to yield a corresponding averaged, or xe2x80x9csmoothedxe2x80x9d, signal, wherein amplitude randomness is effectively removed. In turn, the processor means may correlate at least the first simple interferometric fringe signal and at least one other signal for the ROI (e.g., obtained/generated pursuant to the corresponding-in-time receipt of thermal emissions from the ROI) to obtain at least one xe2x80x9ccompoundxe2x80x9d interferometric fringe signal employable in the formation of a pixel image of the ROI. The signal that is combined with the first simple interferometric fringe signal may be one of a first thermal emission signal, second thermal emission signal or third thermal emission signal (e.g., collected by a third antenna), or perhaps more preferably, a second simple interferometric fringe signal obtained by combining one of the first and second thermal emission signals with a complex conjugate of a third thermal emission signal. As will be appreciated, it is generally preferable to form a plurality of compound interferometric fringe signals for use in image formation.
By way of example, a first simple interferometric fringe signal (e.g., which correlates thermal emission signals generated by first and second antennas) may be combined with a second simple interferometric fringe signal (e.g., which correlates thermal emission signals generated by third and fourth antennas) to obtain a first compound interferometric fringe signal. Similarly, simple fringe correlations of the first and third thermal emission signals and of the second and fourth thermal emission signals can be further correlated to obtain a second compound interferometric fringe signal. Further, the first and second compound interferometric fringe signals may be further combined in an additional stage. The formation/utilization of simple interferometric fringes and compound interferometric fringes particularly facilitates a sparse aperture imaging arrangement, thereby reducing the number of space vehicles/antennas needed to generate high-resolution images.
In yet a further aspect of the present invention, the processor means may be provided to extract pixel values therefore, (e.g., complex values (i.e., comprising phase and amplitude components) or real amplitude values) from at least one and preferably a plurality of interferometric fringe signals employed for image formation on a per pixel location basis. In turn, the extracted pixel values are employable by the processor means to xe2x80x9cdevelopxe2x80x9d the pixel image of the ROI. By way of primary example, for each different interferometric fringe signal employed for ROI pixel image formation, the processor means may provide for the corresponding application of a plurality of different matched filters (e.g., corresponding with each of a plurality of pixel locations for the ROI pixel image to be formed) to obtain a plurality of extracted pixel values in corresponding relation to each of the plurality of pixel locations. In turn, the pixel values corresponding with each given pixel location may be utilized to form an interferometric image signal (e.g., for each interferometric fringe signal employed for image formation). In one arrangement, for each given one of a plurality of interferometric fringe signals employed, the extracted pixel values for each given pixel location may be combined to obtain a corresponding interferometric image signal . The plurality of interferometric image signals corresponding with the plurality of interferometric fringe signals employed may be merged (e.g., via complex summation and/or simple or weighted averaging) to yield the ROI pixel image. The utilization of separate matched filters for each pixel location and each interferometric fringe signal employed facilitates near-field imaging of an ROI as discussed above.
In view of the foregoing, it will be appreciated that an inventive method may comprise the steps of collecting thermal emission from a planetary surface region of interest (ROI) by a plurality of spaced antennas to obtain a corresponding plurality of thermal emission signals. Following collection, the method further includes the step of first combining at least a first thermal emission signal with a complex conjugate of at least a second thermal emission signal to obtain at least a first simple interferometric fringe signal. Preferably, a plurality of different simple interferometric fringe signals are formed from different pairs of collected thermal emission signals, wherein the data comprising one of each such pairs may be time-shifted to maintain mutual coherence. Each simple interferometric fringe signal may be low-pass filtered to remove undesired high-frequency components and otherwise yield an averaged signal.
In one aspect, the inventive method may further comprise the step of second combining at least a first simple interferometric signal with another signal for the ROI (e.g., a signal generated from corresponding-in-time thermal emissions from the ROI) to obtain at least a first compound interferometric fringe signal. In this regard, the second combining step may provide for the combining of one of the first and second thermal emission signals with the complex conjugate of a third thermal emission signal to obtain a second simple interferometric fringe signal. As such, the noted first compound interferometric fringe signal may be generated by combining a first simple interferometric fringe signal with one of (i) the second simple interferometric fringe signal, and (ii) one of said first, second and third thermal emission image signals. Preferably, a plurality of different compound interferometric fringe signals are formed. As noted, the formation/use of one or more compound interferometric fringe signals in image formation facilitates the use of a sparse aperture arrangement.
In another aspect the inventive method may provide for (i) collecting thermal radiation at the collection antennas over a predetermined frequency bandwidth of about 1 MHz to 1 GHz, and (ii) positioning at least two of the collection antennas to define an angle of at least about 2xc2x0 between their respective center imaging axes (e.g., to define a near-field imaging arrangement). Further, for at least a first interferometric image signal, and more preferably for each given one of a plurality of interferometric fringe signals (e.g., simple and/or compound), the inventive method may include the step of applying a different matched filter corresponding with each of a plurality of pixel locations to interferometric signal data to extract a plurality of pixel values (e.g., complex values or real amplitude values) corresponding with each of said plurality of image pixel locations. Then, for each interferometric fringe signal employed, the inventive method may include the step of combining the pixel values corresponding with each of the plurality of pixel locations (e.g., summing) and utilizing the combined pixel values to obtain an interferometric image signal employable in the formation of the pixel image of the ROI. Where matched filtering is applied to a plurality of different interferometric fringe signals, the resultant plurality of interferometric images signals may be merged to yield the ROI pixel image. By way of example, such merging may provide for the averaging or weighted averaging of the different interferometric images in generating the ROI pixel image.
Additional aspects and advantages of the present invention will be readily apparent to those skilled in the art after consideration of the further description that follows.